You make it sound like this is the fault of the person who is not on Facebook, as opposed to the fault of the people who chose to use a system that has zero interoperability with other systems. From where I sit, the problem is you for choosing to use a closed system to organize social events. The problem is you for allowing Facebook to control your social life in this fashion.

Making someone into an outcast because they do not use your favorite social networking system is the problem here.